1. Field of the Invention
This invention relates generally to phase-locked loop circuitry and, more particularly, to a system and method of using different loops, with a shared loop filter, for the tasks of frequency acquisition and phase tracking.
2. Description of the Related Art
It is well known to acquire and track a transmitted carrier signal at a receiver through the use of frequency and phase-locked loops (PLLs) using circuit elements such as voltage controlled oscillators (VCOs), frequency dividers, frequency filters, phase detectors, and reference frequency sources. PLL design techniques using such circuit elements are discussed in Phaselock Techniques, Floyd Gardner, John Wiley and Sons, Inc., N.Y., 1966 and in Frequency Synthesis Theory and Design, Vadim Manassewitsch, John Wiley and Sons, Inc., N.Y., 1987.
Wide bandwidth loops are used in frequency acquisition. The wide bandwidth gives the loop sufficient speed to acquire a carrier signal with an unknown frequency, as the signal exists in a range of potential frequencies. However, the wide loop bandwidth also opens the loop to the noise components which are added on top of the signal generated by the VCO. A noisy oscillator signal is not always sufficient to demodulate the information carried in the transmitted signal, especially if the ratio of the transmitted signal to noise floor is small.
To remove noise from the loop, and therefore from the oscillator output signal, a narrowband loop is used. Such a loop is not responsive enough to acquire a carrier signal in a wide range of frequencies, but it is able to track a carrier signal once acquired, as long as the changes in the carrier signal are small or gradual. Therefore, it is conventional for some receivers to use a dual-loop system to acquire and track a transmitted signal.
The phase detector, oscillator, filter, and divider components all have associated gains that contribute to the overall loop gain. Conventionally, the oscillator has a gain that is constant with respect to its input frequency, and the conventional digital phase detector has a gain that varies with respect to phase. Since a frequency change in the carrier signal input to the conventional phase detector remain proportional to the frequency in the feedback path, the phase detector gain can be said to have a gain that is independent of frequency. A loop filter has a gain that is inversely proportional to frequency. While the divider does not directly affect the loop gain, a higher divisor (lower frequency) acts to decrease the loop bandwidth.
Although first-order PLL loops are unconditionally stable, their design is not always practical, as the delays associated with the divider and phase detector circuits must be factored into the loop transfer function. Hence, many loops are designed to be second-order. Such circuits are useful in tracking or frequency synthesis where the loop must selectively track frequencies within a relatively narrow band. Often the frequency divider is made variable to select the frequency. It is very desirable that such a loop work across the entire range of frequencies without having to adjust component values. Advantageously, many loop designs are autoscaling in the sense that the loop bandwidth varies in proportion to the carrier signal frequency, as the divisor ratio must be varied as the carrier signal frequency changes to supply the same oscillator signal frequency. For example, at higher carrier frequencies a smaller divisor is needed and the bandwidth is relatively large. At lower carrier frequencies the divisor is larger and the bandwidth is relatively narrow. This gain-bandwidth compensation permits the same components to be used across the entire range of frequencies.
Since second-order loops are not unconditionally stable, there can be problems in attempting to make a circuit selectable track carrier signals over a very wide range of frequencies. The above-mentioned circuit elements can be changed or modified to affect the overall loop gain. Many designers find that it is easiest to change the loop bandwidth by switching in different resistor and capacitor components of the filter. Often the filter is designed as an operational amplifier, where the amplifier gain characteristics and the feedback circuit components control the filter response. It is even more difficult to design a selectable filter so that the loop can operate wideband to acquire a signal or narrowband to track a signal.
It is well known to use a so-called bang-bang phase detector in a PLL loop. These detectors permit a loop to maintain a constant gain despite changes in the carrier signal data rate. As such, they are useful in a acquisition loop where it is desirable to have a uniform loop gain across a wide frequency range. A loop using a bang-bang detector is also useful because it can acquire a carrier signal without the use a known reference frequency standard. However, a wideband loop using a bang-bang phase detector is not desirable for generating a narrowband, low noise signal for tracking, as it lacks the autoscaling feature, mentioned-above, which scales the loop gain bandwidth to the data rate.
It would be advantageous if a circuit could be designed, using the same loop filter to both track and acquire a carrier signal over a wide range of frequencies.
It would be advantageous if a dual-loop system could be designed using a common loop filter, one for tracking frequencies, and one for acquiring frequencies.
It would be advantageous if the above-mentioned dual-loop system were able to maintain a consistent loop gain in the acquisition mode. Further, it would be advantageous if the loop bandwidth could be scaled to the data rate of the carrier signal, once the tracking mode was selected.
Accordingly, an integrated circuit (IC) system for acquiring and tracking the frequency of a carrier signal is provided. The system comprises an acquisition loop having a fixed or constant first loop gain, independent of data rate, and a tracking loop having an autoscaling second loop gain that is responsive to the data rate of the carrier signal.
The acquisition loop includes a bang-bang phase detector, having a gain proportional to input data rate, to accept an acquisition feedback signal and a reference signal, to supply a first phase detector signal. An oscillator is connected to the bang-bang phase detector to accept the first phase detector signal. The oscillator supplies an oscillator signal with a frequency that is proportional to the voltage level. A first divider divides the oscillator signal by the first divisor to supply the first divisor quotient to the bang-bang phase detector.
In one aspect of the invention, the reference signal to the bang-bang phase detector is the carrier signal itself, so that the loop is self-acquiring. Alternately, a clock provides a clock standard signal with a predetermined frequency as the reference.
The tracking loop includes a Hogge phase detector, having a gain responsive to phase, but independent of the data rate of the input signals. The tracking lop also includes a second divisor and an oscillator. The oscillator is shared with the above-mentioned acquisition loop. A multiplexor (MUX) is connected to the output of the second phase detector and the output of the first phase detector. The MUX selects the phase detector output to be provided to the oscillator and, therefore, which loop is being operated.
A dual-loop method for acquiring and tracking a carrier signal with an IC device is also provided. The method comprises: acquiring a carrier signal with a fixed first loop gain, independent of the carrier signal data rate; and, following acquisition of the carrier signal, tracking the carrier signal with an autoscaling second loop gain that is a function of the carrier data rate.
With respect to the above-described dual-loop system, acquiring the carrier signal includes comparing the acquisition feedback signal to the carrier signal, and amplifying the resultant comparison by a bang-bang phase detector gain, which has a gain proportional to the data rates of the carrier signal. Following the acquisition of the carrier signal, the method switches from the first loop gain to the second loop gain. Likewise, tracking the carrier signal includes comparing the tracking feedback signal to the carrier signal and amplifying the resultant comparison by a Hogge phase detector gain which is independent of the carrier signal data rate.
In some aspects of the invention, the acquisition of the carrier signal is accomplished without a reference. Alternately, the method further comprises: accepting a clock standard signal having a predetermined frequency. Then, the acquisition of the carrier signal includes the reference signal as the carrier signal until acquisition occurs.